[core] regression - the new `LambdaClock` and `Instant` breaks HTTP headers sent by LocalServer

[sebsto]

The change recently introduced by #540 (remove dependency on DispatchWallTime) introduced a regression.

The Lambda runtime client crashes because of "missing required headers" (AWSLambdaRuntime.LambdaRuntimeError.Code.nextInvocationMissingHeaderDeadline) when calling the Lambda Local Server for testing.

When the runtime client calls HTTP GET /next, it checks for presence of mandatory server headers. One of them is Lambda-Runtime-Deadline-Ms which is expected to be an EPOCH number (see https://docs.aws.amazon.com/lambda/latest/dg/runtimes-api.html#runtimes-api-next)

The newly introduced LambdaClock.Instant doesn't generate the correct value, probably a lack of CustomStringConvertible. The object is rendered as string instead of the EPOCH value.

Lambda-Runtime-Aws-Request-Id: 21235745116708; Lambda-Runtime-Invoked-Function-Arn: arn:aws:lambda:us-east-1:-6648:function:custom-runtime; Lambda-Runtime-Trace-Id: Root=1-68aaeb5d-86cd1c934bb9178e8c7e6ec0;Sampled=1; Lambda-Runtime-Deadline-Ms: Instant(instant: 1756032737492); Content-Length: 26

Instant is not rendered correctly.

Instant(instant: 1756032737492)

instead of

1756032737492

[sebsto]

Fixed by #552